Update CI badge to use Actions

README.md

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-    <a href="https://badge.fury.io/rb/just-the-docs"><img src="https://badge.fury.io/rb/just-the-docs.svg" alt="Gem version"></a> <a href="https://travis-ci.com/pmarsceill/just-the-docs"><img src="https://travis-ci.com/pmarsceill/just-the-docs.svg?branch=master" alt="Build status"></a>
+    <a href="https://badge.fury.io/rb/just-the-docs"><img src="https://badge.fury.io/rb/just-the-docs.svg" alt="Gem version"></a> <a href="https://github.com/pmarsceill/just-the-docs/actions"><img src="https://github.com/pmarsceill/just-the-docs/workflows/CI/badge.svg" alt="Build status"></a>
 </p>
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